Hitherto, known processes for separating p-xylene from a hydrocarbon mixture by crystallization include: (a) a process for crystallizing p-xylene comprising cooling p-xylene containing hydrocarbon liquid with a refrigerant through a heat-transfer face of an evaporator; this is referred to as the indirect cooling process or well scraping process, and is described, for example, in U.S. Pat. No. 283,383; and (b) a process for crystallizing p-xylene comprising cooling by a direct cooling process and a circulation stream process, namely, mixing a p-xylene containing hydrocarbon liquid with an inert liquid refrigerant and volatilizing the mixed refrigerant in the hydrocarbon liquid to cool the hydrocarbon liquid, described, for example, in U.S. Pat. No. 2,533,232. In process (a), the size of p-xylene crystals is small. Consequently, the yield is low because the separation efficiency in the subsequent filtration or centrifugal separation step is undesirably low. Further, since the mother liquor is occluded in the resulting crystal cake (i.e., cavities in the cake are filled with the mother liquor), the purity of p-xylene in the separated cake is low and, consequently, subsequent purification is difficult to achieve.
In the direct cooling-circulation process of (b), a special circulation apparatus is required, because it is necessary to create a large circulation stream. Moreover, it has the fault that a plurality of apparatus must be used in order to produce p-xylene in a large volume, because a single apparatus has a very limited capability.